1. Field of the Invention
This invention relates to a curable resin composition which, in a multilayer printed circuit board having conductor circuit layers and insulating layers alternately built up or superposed on a substrate, is capable of forming insulating layers excelling in adhesiveness with conductor layers and also excelling in resistance to heat, a multilayer printed circuit board manufactured by using the curable resin composition, and a method for the production thereof.
2. Description of the Prior Art
For the production of a multilayer printed circuit board, a method which comprises laminating a plurality of circuit boards having a prescribed circuit patter-n formed in advance thereon through the medium of prepregs as adhesive insulating layers, pressing the laminate thus obtained, drilling and making plated through-holes to interconnect the circuits in the component layers has been heretofore known to the art (lamination pressing method). The lamination pressing method, however, necessitates use of production facilities which are very voluminous and highly expensive and incurs difficulty in forming fine patterns because the copper used in plating through-holes enters the outer layers and adds to the thickness of copper deposit.
In recent years, strenuous efforts are being continued for the development of a multilayer printed circuit board having conductor layers and organic insulating films alternately built up or superposed on a substrate (build-up method) for the purpose of overcoming such problems as are attendant on the conventional method mentioned above. In this build-up method, such techniques as vacuum deposition and sputtering are generally adopted for the formation of conductor layers on insulating layers. The method, however, is at a disadvantage in betraying inferior productivity and entailing high cost.
In another method of producing a printed circuit board, a xe2x80x9cfully additivexe2x80x9d technique is employed. As one of the xe2x80x9cfully additivexe2x80x9d method, Japanese Patent Publication No. 4-6116 discloses a method which forms a conductor layer by electroless copper plating through the medium of an adhesive layer formed on an insulating substrate. In this method, an adhesive agent composed mainly of rubber, a filler, and a thermosetting resin is used for improving the adhesive force to be exhibited by a plating film to the adhesive layer. This adhesive agent is applied to the insulating substrate and allowed to harden thereon to produce the adhesive layer. Then, the adhesive layer is treated as with a mixture of chromate and sulfuric acid and cleansed with hot water to give rise to a surface construction in which the depressions formed in consequence of the removal of filler particles and the protrusions of rubber particles exposed in consequence of the removal of thermosetting resin are uniformly distributed. This method, however, is at a disadvantage in suffering the rubber component to persist within the adhesivie layer and cause degradation of such properties as resistance to heat and electrical insulating properties.
An object of the present invention, therefore, is to provide a curable resin composition which, in the production of a multilayer printed circuit board having conductor layers severally bearing a prescribed circuit pattern and resinous insulating layers alternately built up or superposed on a substrate, accomplishes highly satisfactory bonding between the component layers by virtue of an epoxy resin excelling in resistance to heat and electrical insulating properties without requiring the resinous insulating layers to use a rubber component as an essential component and enables the conductor layers to exhibit necessary adhesiveness to the insulating layer.
Another object of the present invention is to provide a multilayer printed circuit board having resinous insulating layers and conductor layers alternately built up on a substrate and joined each other with highly satisfactory adhesive strength and enabling the resinous insulating layers to excel in various properties such as resistance to heat and electrical insulation which are expected from a printed circuit board and a method capable of producing the multilayer printed circuit board inexpensively with high productivity.
To accomplish the objects mentioned above, in accordance with one aspect of the present invention, there is provided a curable resin composition which comprises (A) epoxy resins and (B) an epoxy resin curing agent as essential components thereof, the epoxy resins (A) comprising (A-1) a bisphenol A type epoxy resin having an epoxy equivalent of not less than 400 and (A-2) an epoxy resin having an epoxy equivalent of less than 400 and containing at least two epoxy groups in the molecular unit thereof in the ratio of the epoxy resins (A-1:A-2) in the range of from 30:70 to 90:10 by weight so that a cured coating film formed of this resin composition can be partially decomposed or dissolved with a coarsening agent and consequently endowed with an undulating coarsened surface, and the epoxy resin curing agent (B) containing at least two active hydrogen atoms in the molecular unit thereof.
In a preferred embodiment of the present invention, the curable resin composition is allowed to contain, in addition to the essential components mentioned above, a rubber component in a proportion of not more than 40 parts by weight, based on 100 parts by weight of the epoxy resins (A) and optionally further contain a filler capable of being decomposed or dissolved by a coarsening agent in a proportion of less than 70 parts by weight, desirably not more than 50 parts by weight, based on 100 parts by weight of the epoxy resins (A) mentioned above.
In accordance with another aspect of the present invention, there is further provided a method for the production of a multilayer printed circuit board having resinous insulating layers and conductor layers severally bearing a prescribed circuit pattern sequentially superposed on a conductor layer of a circuit board having a prescribed circuit pattern formed in advance thereon, wherein the formation of resinous insulating layers and conductor layers comprises the steps of (a) forming the resinous insulating layer by coating a circuit board with the curable resin composition according to the present invention and subjecting the resultant applied layer to a thermal curing treatment, (b) treating the surface of the resultant resinous insulating layer with a coarsening agent thereby imparting an undulating coarsened surface to the resinous insulating layer, and (c) forming a conductor layer on the coarsened surface of the resinous insulating layer. This method for the production of the multilayer printed circuit board, in ore concrete embodiment, further comprises the steps of boring prescribed through-holes in the circuit board after the formation of the outermost resinous insulating layer, treating the surfaces of the resinous insulating layer mentioned above and the through-holes with a coarsening agent thereby imparting coarsened surfaces thereto, and thereafter forming the outermost conductor layer on the coarsened surface of the outermost resinous insulating layer.
Preferably, the coating of the resinous insulating layer with the conductor layer is effected by electroless plating and/or electrolytic plating and at least one member selected from the group consisting of oxidizing agent, alkali solution, and organic solvent is used as the coarsening agent.
When the screen printing technique, for example, is used in coating the conductor layer of the circuit board with the resinous insulating layer mentioned above, it will be difficult to obtain an applied film of desired thickness by a single coating work. In the case of this sort, the curable resin composition of the present invention may be applied repeatedly or the resinous insulating layer may be formed preparatorily with an ordinary solder resist possessing adhesiveness to the conductor layer, desirably a curable resin composition composed mainly of an epoxy resin and an epoxy resin curing agent, and then coated with the curable resin composition according to the present invention. Thus, the applied coating consequently formed functions as an adhesive layer for the conductor layer to be formed afterward.
By the aforementioned method, there can be manufactured with high productivity a multilayer printed circuit board which has a resinous insulating layer and a conductor layer bearing a prescribed circuit pattern sequentially superposed on a conductor layer of a circuit board having a prescribed circuit pattern formed in advance thereon, in which the resinous insulating layer is formed of a cured coating film of the resin composition according to the present invention, the surface of this resinous insulating layer which defines the interface with the conductor layer to be applied thereon is formed in an undulating rough surface by a coarsening treatment, and the conductor layer is joined to the resinous insulating layer through the medium of the rough surface thereof.